1. Field of the Invention
The present invention relates to a device for connecting an optical fibre. More precisely, the invention relates to a connector for optical fibres and a device for connecting one or more optical fibres to a respective connection component. The invention also relates to a method for manufacturing a connector for an optical fibre, a method for terminating a fibre-optic cable, a fibre-optic communication line and a distribution network comprising at least two fibre-optic communication lines.
Throughout the present description and the subsequent claims the term “connection component” will be used to generally indicate an optical component, such as for example an optical fibre or a lens, or a opto-electronic components, such as for example an optical source (laser) or a photo-detector (or photo-receiver or photo-diode). For the sake of simplicity, henceforth reference will be explicitly made to the optical connection between optical fibre and opto-electronic component, i.e. a device which carries out an optical-electrical and/or an electrical-optical conversion, such as an optical source or a photo-detector, respectively; the same principles can, however, be applied to the case of a connection between two optical fibres, or between an optical fibre and a lens.
2. Description of the Related Art
Currently, in the field of telecommunications, optical technology is mainly used for long-distance transmission of optical signals using the known properties of wide band provided by optical fibres. On the contrary, for distributing signals to a plurality of users (such as, for example, television and/or analogue and/or digital telephone signals) and for transmitting digital data between electronic equipments (such as, for example, the Personal Computers of a LAN network) the technology which is mainly used makes use of electrical cables, such as, for example, coaxial or copper duplex cables.
Nevertheless, electrical cables have a relatively narrow band and they are becoming a bottleneck with respect to the band of signals to be transmitted. Moreover, they exhibit problems of electromagnetic interference, of impedance matching and they are difficult to be inserted into the appropriate raceways of a building since they are stiff. In addition, being bulky, they significantly reduce the number of cables which can be inserted into a raceway. Moreover, due to electrical safety requirements, they require the arrangement of separate raceways from those used for distributing electrical energy.
Thus the research is moving towards the possibility of using optics not just in the long-distance transmission of signals, but also in the signal distribution networks from a common branch point to a plurality of user apparatuses. Indeed, fibre-optic cables are suitable for being inserted into the appropriate raceways of a building since they are not too bulky, they are flexible, light, free from electromagnetic interference and they have low bending losses. In addition, they are suitable for being inserted into the same raceways which are used for distributing electrical energy. Moreover, optical fibres have a potentially very large band, low attenuation values and they are transparent to the bit rate, to the format and to the transmission code.
In addition, amongst the various types of optical fibres, single-mode optical fibres are preferable since they are much less sensitive to bending losses, they are per se less expensive, more robust, they have lower absorption losses, they are suitable for being used for a wavelength division multiplexing (or WDM) transmission and they have a wider band, making a signal distribution network easily expandable.
Nevertheless, for connection to electronic apparatuses, fibre-optic cables require the use of opto-electronic components to convert the electrical signals into corresponding optical signals and vice versa.
The conversion of an electrical signal into a corresponding optical signal is conventionally carried out by modulating the intensity of a light emitted by an optical source. The modulation is associated with the information transported by the electrical signal and the conversion of an optical signal into a corresponding electrical signal is conventionally carried out by means of a photo-detector.
The connection of an optical cable to an optical source and/or to a photo-detector is conventionally carried out by means of an optical connection device. Typically, an optical connection device is a device comprising two parts (henceforth referred to as connector and receptacle) which can be repeatedly connected to and disconnected from each other and which must be attached to one end of the optical cable and to the optical source or the photo-receiver, respectively.
In the specific case of mutual connection between two optical fibres, each of the two fibres is associated with a respective connector and the optical coupling is achieved by mutually connecting the two connectors by means of an intermediate connection element. Throughout the present description and the subsequent claims, the term “receptacle” is to be understood to also include such an intermediate element.
For example, an optical cable installation suitable for connecting, inside a building, a user apparatus to a central distribution apparatus (located, for example, in an office or in a flat and, respectively, in the cellar or in the loft), requires the implementation of the following steps: passing the optical cable along an appropriate raceway in the building; cutting the optical cable according to the desired length; clamping two connectors to the two ends of the cable, in the proximity of the user apparatus and of the central apparatus, respectively; optionally clamping the optical source and/or the photo-detector (if the optical source and the photo-detector are not already provided in appropriate receptacles) to two receptacles, in the proximity of the user apparatus and of the central apparatus, respectively; and, finally, connecting each optical connector with the respective receptacle for removably connecting the optical fibre with the source and/or the photo-detector, in the proximity of the central apparatus and of the user apparatus, respectively.
Nevertheless, conventional optical connection devices are disadvantageous to be used since an extremely precise alignment (in the range of micrometres) between optical fibre and optical source and/or photo-detector is required: such an alignment operation, apart from requiring a long execution time and being very difficult to carry out directly on site, must be carried out by highly specialised personnel, resulting therefore in high installation costs. Moreover, in order to achieve an extremely precise alignment, a highly precise fibre cutting operation must be carried out beforehand: such a cut must, indeed, be such as to define a free end of the fibre with a frontal face which is as flat and uniform as possible. To increase the precision of the optical alignment, such a face is typically also subjected to a lapping process using particular devices which are suitable for this purpose.
These aspects are even more relevant for single mode fibre-optic cables since a single mode optical fibre, having a core with a very small diameter (typically, 7–10 μm compared to that, for example, of 50–70 mm of a multimode glass optical fibre or to that larger than 100 μm of a multimode polymeric optical fibre) requires an alignment with the opto-electronic component with a precision in the order of one or few micrometres.
Throughout the present description and the subsequent claims we will often use the expression “portion of bare fibre”. Such an expression is used to indicate a portion of optical fibre from which the acrilate coating layer has been removed so as expose the glass surface of the cladding.
Devices for realising a non-permanent optical connection between an optical fibre and an opto-electronic component (or another optical fibre) are known. The Applicant has studied such devices and has found a number of drawbacks.
WO 97/23797 discloses a device for connecting an optical fibre and an opto-electronic component or another optical fibre, comprising a connector associated with one end of a fibre-optic cable and a receptacle intended to receive said connector to realise the optical coupling between two optical fibres (or between the optical fibre and the opto-electronic component). The connector comprises an outer casing inside of which a fibre-holding device provided with a groove for housing the fibre is provided. The casing has an opening and a door for closing said opening: the door is intended to move when the connector is associated with the receptacle to allow the optical coupling between the fibre and the opto-electronic component or the other fibre be achieved. The receptacle comprises an alignment device intended to be inserted into the connector through the aforementioned opening to take in the free end of the bare fibre so as to realise the optical coupling. The alignment device comprises, in particular, a V-groove inclined with an angle of about 42° with respect to the longitudinal axis of the fibre of the connector. When the connector is completely inserted into the receptacle, the free end of the fibre is housed in the V-groove and is in contact with the face of the other fibre (or with the active surface of the opto-electronic component housed in the receptacle), thus realising the optical coupling. The device is designed in such a way that when the fibre of the connector comes into contact with the fibre of the receptacle (or with the active surface of the opto-electronic component housed in the receptacle), the fibre of the connector arches so as to define a radius of curvature less than 7.6 mm. Before carrying out such a coupling operation the bare fibre is lapped through a suitable device which is described and illustrated in the same application. Such a device has an opening for allowing the insertion of the connector; with the connector inserted, a lever pushes the bare fibre out from the outer casing of the connector in such a way that it can come into contact with a tape intended to realise the lapping process. The Applicant has noted that, in a device of the type described above, the fibre is subjected to high stresses due to the great angle of curvature (small radius of curvature) of the fibre itself when the connector is inserted into the receptacle; said stresses can be such as to cause the breaking of the fibre and in any case they reduce the reliability in operation (this is made worse by the fact that, since the connector is intended to be repeatedly inserted into and removed from the receptacle, the fibre is also subject to strain stresses). The Applicant has, moreover, observed that, with a device of the type described above, it is necessary to determine with particular attention the length of the bare fibre before carrying out the lapping process: indeed, the length must be such as to allow the contact of its end face with the lapping tape when the connector is inserted into the lapping device. It is, therefore, necessary, before carrying out the lapping process, to cut the bare fibre to a predetermined length according to the specific lapping instrument used.
U.S. Pat. No. 4,756,591 discloses a device for connecting an optical fibre and an opto-electronic component, comprising a support upon which a high precision V-groove is formed, said groove being intended to house a portion of the bare end of an optical fibre. The opto-electronic device is fastened onto the support at one end of the V-groove, with its active surface turned towards the groove. This groove is sized in such a way that, when the portion of bare end of the fibre is positioned therein, the face of the free end of such a portion of fibre is aligned with the active surface of the opto-electronic device. The optical coupling between optical fibre and opto-electronic device is carried out by positioning the end portion of the fibre in the groove and bringing the face of the free end of such a fibre portion into contact with the active surface of the opto-electronic device. The aforementioned end portion of the fibre is held in position into the groove by means of suitable fibre blocking elements arranged above the support and upon which a vertical elastic force acts; such elements are activated after having realised the optical coupling between fibre and opto-electronic device. The Applicant has observed that, with a device of the type described above, particular attention both to the sizing and manufacturing of the V-groove, and to the fastening of the opto-electronic device onto the support must be paid in order to achieve a very precise optical alignment between fibre and opto-electronic component; it is, therefore, necessary that the support is manufactured in a material suitable for allowing precise operations to be carried out (such as silicon treated through etching). Moreover, it is necessary to pay particular care to the positioning of the portion of bare fibre into the V-groove: such a portion of fibre, indeed, does not have the protective acrilate layer and is thus particularly exposed to possible knocks or contamination, which can cause damage and/or breaking of the fibre.
U.S. Pat. No. 4,767,180 discloses a device for connecting two optical fibres, each fibre being associated with a respective connector and being intended to be optically coupled with the other fibre in a suitable coupling bush. Each connector houses a fibre-holding element from which an end portion of bare fibre protrudes. Said portion passes through an appropriate perforated channel and arrives at the free end of the connector, as far as it protrudes out of the connector for a predetermined length. Each connector is intended to be housed in a respective seat provided in the coupling bush. Between the seats a guide element is interposed upon which a high precision V-groove is formed; said groove is intended to house and align the bare end portions of the two fibres when the two connectors are inserted into the respective seats. The optical coupling between the two fibres into the V-groove is realised by inserting the two connectors into the respective seats and making them mutually abut. The abutment surfaces of each connector are defined at the end of two extensions provided laterally with respect to the end portion of the fibre; each of these lateral extensions is intended to be inserted into a respective seat provided in the bush on a side of the V-groove. A locking element arranged above the bush, when activated, holds the two ends of fibre coupled in the V-groove. In such a device, the end portion of the fibre intended to be housed into the V-groove to realise the optical coupling always protrudes from the container for a predetermined length, also when the connector is not associated with the coupling bush. The Applicant has observed that, particularly during the connector maintenance operations and during the initial steps for connecting the connector to the bush, such a portion of bare fibre is particularly exposed to possible knocks or contamination, which can cause damage and/or breaking of the fibre.
U.S. Pat. No. 4,218,113 discloses a device for connecting two or more optical fibres, comprising a connector associated with one end of a fibre-optic cable so as to let a predetermined length of end portion of bare fibre project. The fibre is housed in the connector into an appropriate seat provided in a cylindrical support which can slide inside an outer protection cylinder. The connector is intended to be associated with a sleeve provided with a calibrated hole intended to receive the end portion of fibre which protrudes from the connector to realise the optical connection with another end portion of fibre also intended to be housed in the calibrated hole on the opposite side to the previous one. The hole has a diameter which is calibrated so as to realise the optical alignment between the fibres. The optical connection is realised by associating the connector with the sleeve: the sleeve has a cylindrical surface which protrudes out for a predetermined length and which, when penetrating into the connector's protection cylinder, pushes the support backwards (opposing the elastic force of a spring); the end portion of fibre is thus guided and housed into the sleeve's calibrated hole thus realising the optical coupling with the other fibre. In such a device the end portion of the fibre intended to be housed in the calibrated hole to realise the optical coupling protrudes from the connector for a short length which must be controlled with precision during the assembly of the cable to the connector. The Applicant has observed that this prevents the execution of precise cutting and possible lapping process; moreover, such operations require the use of an instrument which is suitable and specifically provided for such a connector.
U.S. Pat. No. 5,348,487 discloses a connector for optical fibres comprising, at one of its free ends, a foldaway lid intended, when the connector is not inserted into the receptacle, to close the end of the connector so as to protect the free end of the bare fibre housed inside of it. When the connector is inserted into the receptacle the cover is folded into open position so as to allow the full insertion of the connector into the receptacle and, thus, to realise the optical coupling. In such a device the fibre never protrudes from the connector and therefore is never fully exposed when the connector is not inserted into the receptacle. However, the Applicant has observed that when using such a device, the cutting, and possibly lapping, process of the fibre must be carried out before the fibre is associated with the connector: in such a case it is necessary to handle the bare fibre with extreme care in order to avoid damaging or breaking of the fibre.
U.S. Pat. No. 5,732,174 discloses a device for connecting two or more optical fibres. Each fibre is associated with a connector intended to be inserted into a receptacle. Each connector comprises an outer casing, a fibre protection cover slidably mounted inside the outer casing, a bush slidably mounted inside the protection cover and a fibre-holding device housed within the bush and integral therewith. The cover is mobile between a first work position (connector removed from the receptacle) wherein it houses the end portion of fibre intended to be optically coupled, and a second work position (connector inserted in the receptacle) wherein it completely uncovers an end part of such an end portion of fibre. The cover comprises a front surface provided with a calibrated hole having a diameter which is slightly larger than the diameter of the bare fibre (0.05 mm larger than the diameter of the fibre): during the insertion of the connector into the receptacle, the cover is pushed into the second operational position and the end part of the end portion of fibre projects from the cover through the aforementioned hole and positions itself in a precision V-groove provided in the receptacle, where the optical coupling with the other fibre takes place. In the connection operation, the hole in the cover guides the fibre towards said V-groove. The Applicant has observed that, in a device of the type described above, it is necessary to pay particular attention both at the sizing of such a hole (in order to ensure the function of guiding the fibre and, at the same time, to avoid damaging the bare fibre during the repeated movement of the cover from the first to the second operational position and vice versa), and at the assembly and maintenance of the connector (in order to realise a precise alignment between fibre and hole).
The Applicant has considered the problem of manufacturing a device for connecting an optical fibre wherein the fibre of the connector does not undergo high stresses and wherein the connector, still contributing to the realisation of a high precision optical alignment when inserted into an appropriate receptacle, is not expensive, structurally simple so as to be assembled directly on site, and at the same time such as to allow the risks of damage and/or breaking of the fibres during the maintenance operations and when working to be reduced.